1. Field of the Invention
The invention described herein relates to the field of circuit breakers, and more particularly, to the field of rating plugs for circuit breakers having electronic sensors or trip units.
2. Brief Description of Related Art
Patented disclosures of circuit breakers having electronic trip units (“ETU”s) and removable rating plugs for setting the circuit breaker ampere rating may be found, for example, in U.S. Pat. Nos. 4,672,501; 4,181,922; 6,804,101 and 6,678,135.
Circuit breakers are widely used to protect electrical lines and equipment such as cables, motors, and other loads in an electric circuit by measuring an electrical current. The circuit breaker monitors the current through an electrical conductor and “trips” to open the electrical circuit and thus interrupt current flow through the circuit provided that certain predetermined criteria are met.
An electronic trip unit is a device that is conventionally used in conjunction with a circuit breaker to control the circuit breaker's current (and, or voltage) versus time trip response. In this capacity, the trip unit typically receives information relating to current through the circuit breaker via current sensors, such as current transformer (“CT”) and/or Rogowski-type current sensors, and processes the information to provide feedback to a user or to provide trip functionality to the associate circuit breaker. When the sensed current exceeds a pre-defined threshold, the trip unit issues a trip signal, in some cases after a predetermined time delay. Generally, the trip signal is communicated to a solenoid, such as a circuit breaker flux shifter or other suitable device, configured to cooperate with a tripping mechanism to separate the circuit breaker contacts and interrupt the current in the protected circuit.
The time versus current trip characteristics are, in part, a function of a maximum continuous current permitted by the circuit breaker. The predetermined time delay and issuance of the trip signal is an inverse function of the magnitude of the sensed current. For very large magnitude overcurrents, such as would be produced by a fault, a central processing unit (“CPU”), such as a microcomputer, of the trip unit is also programmed to issue a trip signal instantaneously.
The circuit breaker may, of course, also be used to monitor voltage, and trip in a case of any disturbance in predetermined voltage conditions such as under-voltage, over-voltage, and voltage imbalance conditions.
Conventional circuit breaker tripping criteria includes, for example, the maximum continuous current permitted in the protected circuit. The maximum continuous current the circuit breaker is designed to carry is known as a frame rating or current rating of the circuit breaker. As long as the sensed current remains below a predetermined protection rating (such as long-time, short-time, ground fault, or instantaneous), the breaker will remain not trip.
Typically, the ETU records the current flowing through the circuit breaker or motor overload relay via the current sensors, phase amplifiers and an A/D converter. The current sensors may also provide power to the trip unit. The current through the protected electric power circuit is typically sensed by means of current transformers and a corresponding voltage signal is supplied to a signal processor within the ETU circuit. Conventional current sensing systems for the ETU employ a current sensor in each phase and in the neutral, if used. The corresponding voltage signal is often conditioned by a rating resistor in a rating plug, as described below.
Conventional trip units employ a rating plug having a rating resistor, or set of resistors, comprising predetermined resistor values which set a current rating (i.e., gain) which is the maximum continuous current permitted in the electronic circuit. Typically, the at least one rating resistor can function as a “burden resistor” located in series with the secondary current transfer current, or a resistor in the feedback loop of the gain circuit operation amplifier, or a combination of both. The rating resistor provides an analog voltage gain proportional to the sensed current in the protected circuit. The rating resistor value is selected to provide a predetermined voltage when a current proportional to the maximum, continuous current permitted in the protected circuit passes through it. Thus, the value of the rating resistor accordingly sets the ampere rating of the corresponding circuit interrupter. A common electronic circuit interrupter could therefore operate over a wide range of ampere ratings by merely changing the value of the burden resistor within the electronic trip circuit.
In order to provide for adjustment of the current rating so that the circuit breaker can be used to protect circuits with different maximum continuous currents, it is known to incorporate the rating resistor in a replaceable rating plug that may be selectively inserted into the breaker. Careful selection of the rating plug allows the rating resistor to be selected without requiring customized tailoring of each trip unit circuit for each circuit breaker ampere rating. A common ETU can thus operate over a wide range of circuit breaker ampere ratings by merely changing the rating plug.
In practice, however, since operators of the circuit breakers may employ various types of circuit breakers they may be required to stock large numbers of spare trip units in case a particular trip unit for a particular type of circuit breaker fails. The required stocking of the large numbers of the spare trip units may be expensive in terms of occupied space and associated costs.
The rating plug changes the operating curve for actuation of a breaker having an electronic circuit interrupter, thus changing the ampere rating of the breaker. For safety purposes, the circuit breaker must be properly configured to provide the type of protection judged by the customer or plant engineer to be appropriate. Therefore, modification of the protection rating or the current vs. trip time response curve is very serious matter and should be handled appropriately and in a way that prohibits errors. Not all rating plugs are compatible with all electronic trip units. Therefore, a known problem is to ensure that a rating plug is compatible with the electronic trip unit into which it is to be inserted.
It is important to prevent an electronic circuit interrupter from being inserted within an electrical distribution circuit for which the circuit interrupter is over-rated. It is perhaps equally important not to insert a circuit interrupter within an electric power distribution circuit for which the circuit interrupter is under-rated, as so-called “nuisance-tripping” could occur. It is also important to insure that a circuit interrupter is not inserted within an electric power distribution circuit with no rating plug.
For safety's sake, all electronic trip units with interchangeable rating plugs are required to reject incorrect combinations of rating plugs and trip units. Such rejection is typically accomplished by the placement of pins within the receptacle in the trip unit into which the rating plug is to be inserted. The pins, which are normally located on the sides of the trip unit housing, interfere with protrusions on the side of the rating plug housing. Thus, prevention of installing rating plugs that are not compatible with a specific circuit breaker is prevented by keying the rating plug housing and the rating plug receptacle thereby preventing incompatible rating plugs from being installed in the circuit breaker.
While workable, this prior art system has several drawbacks and disadvantages. One of these is that the interference between pins and protrusions does not occur until the rating plug is almost fully inserted into the trip unit, often resulting in the user mistakenly believing that insertion of the rating plug has been properly completed. Another problem is that the pins are independent elements, i.e., they are not part of the rating plug housing or the trip unit housing, and as such a pin may be removed by someone tampering with the unit, and the user will not know whether a pin should be present or not.